Question 1044259
Imagine a cross section through the vertex and along the vertical symmetry axis and placed on a cartesian system putting the vertex as a minimum, on the Origin.


If 18 inches wide at top opening, and 10 inches deep, a point on the parabola is  (9,10).  The "9" because half of x component goes in one direction and the other half goes the other direction.  


{{{y=ax^2}}}, and use the other one given or described point to find "a".
{{{a=y/x^2}}}
{{{a=10/81}}}
-
{{{highlight(y=(10/81)x^2)}}}



Notice that the question does not involve the focus, although mentioned in the description.  The focus can be found.